In a breakdown voltage measurement system, the height of pulses in a signal is considered. Amplification is generally performed before the pulses are measured. For example, a band-limited amplifier is typically used to increase the magnitude of the pulses. Pulse height is then measured using an analog peak detection system. With reference to FIG. 1, an analog peak detector 2 uses a comparator 4 to charge a capacitor 6 through a diode 8. As long as the voltage on the capacitor 6 is less than the voltage of the pulse, the capacitor 6 is charged by the comparator 4. Once the voltage on the capacitor 6 exceeds that of the pulse, charging is stopped. The output voltage of the analog peak detector 2 at this point is equal to the highest voltage that has occurred on the input 3. When a data acquisition system is used to detect and measure individual pulses, the detector 2 must be reset by discharging the capacitor 6 after a pulse has been measured in order to be ready for the next pulse.
It is difficult to configure an analog peak detector of this type to be accurate. The analog peak detector relies on a non-linear feedback loop, which is dependent very heavily on the characteristics of the peak detection system. The delay around the peak detection system, particularly through the comparator 4 and an output buffer, causes the voltage of the output of the analog peak detector to lag fractionally with respect to the input, resulting in an overshoot on the output. The magnitude of this overshoot is typically non-linear with voltage, thereby limiting the accuracy of the system.
It is also necessary with an analog peak detector to compromise on the capacitor 6 used for peak detection. The voltage on the capacitor 6 tends to "droop" once the comparator 4 has stopped charging the capacitor because of leakage currents in the system. This introduces an uncertainty in the measurement, since the voltage decreases by some amount before the magnitude is measured. This effect can be limited by using a larger capacitance. A larger capacitance, however, requires more current for charging, resulting in a lower rate of change of voltage and limiting the maximum frequency that can be used for the amplifier. Thus, loop delays in the analog peak detection system are increased. A further complication is introduced when a reset is required on the analog peak detector. Charge injection from a reset switch can result in offsets on the output of the analog peak detector 2, which further limits the accuracy of the partial discharge measurement system. Accordingly, a need exists for a voltage measurement system which allows for more accurate peak detection. In addition, a need exists for a breakdown voltage measurement system which overcomes inaccuracies due to the generate of inaccurate and invalid voltage readings following a breakdown in a sample being tested. After a sample being tested breaks down, a voltage reading is indeterminate and invalid for a finite delay period extending from the time of the breakdown until the removal of the high voltage from the sample.